Ion trap for a cathode ray tube



Dec. 13, 1955 J. DE GIER ION TRAP FOR A CATHODE RAY TUBE Filed Dec. 21, 1951 may/070% Pom )7 FIT/I INVENTOR r .m G T e N [Q E G S A 6% n n a EL United States Patent f ION TRAP FOR A CATHODE RAY TUBE Johannes de Gier, E'mtlhoven, Netherlands, assiguor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application December 21, 1951, Serial No. 262,690

Claims priority, application Netherlands January 11, 1951 3 Claims. (Cl. 313-75) With cathode ray tubes for use in television reception, use is frequently made of a device by which the negative ions, which are emitted from the cathode at the same time as the electrons or which are produced in the discharge space, are removed from the electron beam in order to avoid the production of a so called ion spot on the image screen.

This invention relates to cathode ray tubes for use in television reception which comprises a de-ionising device in which the ions and electrons are separated by deflection of the electron beam. The path followed by the ions from the cathode ends at an electrode which will hereinafter be referred to as the ion collector. The electron paths are deflected from this path by a magnetic field.

Cathode ray tubes for use in television reception are known which are provided with such an ion collector, the ions being collected on the wall of a circular cylindrical electrode which is provided with a transverse wall. The latter is arranged at or adjacent the point, at which the cross-sectional area of the electron beam has a minimum value and it has formed in it an opening for the passage of the electrons. An image of this aperture is produced on the image screen by an electron-optical lens. The present invention does not relate to tubes with which this is the case. it relates only to tubes of the type with which the transverse wall lies within the object distance from the reproduction lens, such for example as tubes to which relate Figures 3 and 4 of the British patent specification 505,632. The invention permits of minimising the increase in tube length due to the use of a de-ionising device.

A cathode ray tube according to the invention is particularly contemplated for use in devices having a magnetic reproduction lens for focussing the electrons on to the collecting screen. For producing the magnetic field use may be made of an energising coil which surrounds the tube. As an alternative, use may be made of a permanent magnet. The position of the coil or of the magnet relative to the electrode system comprising the deionising device acts on the beam concentration at the area of the transverse wall of the ion collector. The invention assists in choosing this position at which a maximum quantity of electrons passes through the aperture in the transverse wall, and its object is to ensure that the magnets which produce the deflecting field for deflecting the electrons from the path followed by the ions and which, similarly to the magnets for focussing the beam, are arranged outside to engage with the Wall of the tube do not limit the positioning of the focussing means. According to the invention, the electron path between the cathode and the deflection point of the electron beam starting from the cathode is surrounded at least largely by a tube of ferromagnetic material. This permits the magnets for the electron-deflecting field to be arranged closer to the end of the tube and at a slight spacing from the inflection point, with the result that the 2,727,171 Patented Dec. 13, 1955 part of the neck length of the tube which is available for the arrangement of the focussing coil is extended.

In order that the invention may be readily carried into etfect, a number of examples will now be described in detail with reference to the accompanying drawings, in which:

Fig. l is a sectional View taken on the axis of an electrode system comprising an ion collector, use being made of a separate cylinder of ferromagnetic material;

Fig. 2 shows a side View of this construction;

Figs. 3 and 4 are cross-sectional views similar to Fig. l in which the invention is carried into etfect in a slightly different manner;

Fig. 5 is a perspective view of Fig. 1 showing operation of the invention;

Fig. 6 is a view at right angles of Fig. 4.

Referring to Fig. 1, the electrode system of a cathode ray tube comprises an electron-emitting cathode 1, a control electrode 2, an anode 3 and an accelerating anode 4. The latter comprises two cylindrical parts 5 and 6 which are united at a small angle. The part 5 also acts as an ion collector and comprises a transverse wall 7 in which a small aperture 3 is formed. The axes of the electrodes 2 and 3 and of the part 6 are arranged in line with each other and are set at a small angle with the axis of the ion collector 5, which axis is coincident with the axis of the cathode ray tube. The wall 9 forms part of the tube.

Starting from the cathode, the beam constituted by electrons and ions follows the axis of the electrode system. In the surroundings of the point of inflection 16 at which this axis intersects the tube axis, a constant magnetic field is active. The lines of force of this field are normal to the plane of the drawing and the field is produced by a system of magnets 10 and 11 which are arranged on the outside to engage with the tube wall. When traversing this field the electrons are subjected to magnetic forces whose direction is denoted by the arrow and which bring about the deflection of the electron beam. The paths followed by the ions are not subjected to any appreciable alteration and the ions thus strike the wall of the ion collector 5 and the transverse wall. The magnetic forces acting on the electrons have the effect of causing the axis of the emerging electron beam to be coincident with the axis of the ion collector 5.

The transverse wall 7 is struck not only by ions but also by some electrons. The intention is that a maximum quantity of electrons should pass through the aperture 8 in this wall. However, the electron beam is not sharply defined on the cathode side of the wall 7. Formation of the beam is assisted by the electron lens formed between the anode 3 and the accelerating anode 4. Due to the acceleration of the electrons in the gap between the said two electrodes, their path is curved in a radial direction towards the axis. In addition, the electron density is increased when the field of the magnetic reproduction lens permeates the ion collector. This field is produced upon energisation of the coil 12 and increases the concentrating effect of the electron lens formed between the electrodes 3 and 4. The increase in concentration of the electrons which thus results is affected by the position of the energising coil. In order to cause a maximum part of the marginal rays which surround the beam core proper to permeate the beam section limited by the aperture 8, it must be possible for the field of the coil to extend as far as the inflection point 16 of the electron beam. This has the effect of decreasing the share in the electron current which is intercepted by the transverse wall 7 and of producing an intense contrast between the darker and the light patches of the image projected on to the screen. This arrangement also reduces the required tube length.

Arranged within the cathode ray tube is a cylinder 13 made of ferromagnetic material, for example iron or nickel. This cylinder encloses the electron path starting from the cathode 1 as far as in the proximity of the inflection point of the electron beam. Within this cylinder the electrons consequently move in a space which is magnetically field-free. On leaving the area enclosed by the cylinder they are deflected by a stray field. Due to the cylinder being arranged between the two magnets which produce the deflecting field and which are now displaced from the inflection point of the electrons to the cathode the central field of the said magnets is guided by the cylinder around the electron path. In this case, the field of the magnets is required to be more powerful than when they are arranged in the immediate proximity of the inflection point of the electrons but this does not result in an appreciable limitation. The arrangement of the magnets thus obtained has the effect of extending the space available for the arrangement of the focussing coil and permits the latter to be arranged so as materially to reduce the percentage of the electron current which is intercepted by the transverse wall 7.

Fig. 5 illustrates the operation of the invention with greater clarity. As will be observed, the main field between the magnets and 11 is shown as solid lines with an intermediate dashed line portion. The latter represents the field that would be present in that area in the absence of the ferromagnetic cylinder 13. Due, however, to the presence of the latter, the area enclosed therewithin is substantially fieldless. Note further than the field active at the inflection point 16 and operative to produce the desired beam deflection is the stray field of the two magnets 10, 11.

To a first approximation it will be possible to omit the cylinder 13 and to make the anode 3 of ferromagnetic material. This construction is shown in Fig. 3. It is particularly useful when the part 6 of the electrode 4 is provided at the end adjacent the anode 3 with a diaphragm 14 which limits the permeation of the cylinder 6 by the electric field and thus permits of the part of the cylinder as far as the inflection point of the electrons being greatly shortened. The dashed line circle shown in this figure represents the positions of the magnets 10, 11.

A more efiicacious construction is that in which the diameter of the part 6 of the accelerating electrode 4 is larger than that of the anode 3 and the control electrode 2 and the said part extends as far as the proximity of the cathode 1. This construction is shown in Figs.

4 means of a cylindrical body 1'5 of ferromagnetic material.

What I claim is:

l. A cathode ray tube comprising an envelope having a central axis, a bent electrode system for producing and projecting a beam of electrons at an angle to said central axis to intercept the central axis at a point of inflection, said electrode system including a cathode, a control electrode, and an anode, an ion collector comprising a bent cylindrical tubular electrode surrounding the electron beam and the point of inflection and including a transverse wall element positioned at a point other than that of minimum beam cross-sectional area on the side of said point of inflection remote from said cathode, focussing means surrounding the envelope in close proximity to said point of inflection, magnetic deflection means for aligning the beam with the central axis disposed on the side of said point of inflection adjacent said cathode and between said cathode and point, and a tube of ferromagnetic material disposed within the envelope and surrounding a substantial portion of the space between said point of inflection and the cathode, whereby the stray flux generated by the magnetic deflection means is guided to approximately the area of the point of inflection to deflect the beam and align it with the envelope axis.

2. A tube as claimed in claim 1 wherein the anode and ferromagnetic tube are integral with one another.

3. A tube as claimed in claim 1 wherein the bent cylindrical tubular electrode has a portion comprising the tube of ferromagnetic material extending from the proximity of the point of inflection to the cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,131,192 Schlesinger Sept. 27, 1938 2,149,101 Ploke Feb. 28, 1939 2,181,850 Nicoll Nov. 28, 1939 2,211,613 Bowie Aug. 13, 1940 2,211,61 Bowie Aug. 13, 1940 2,347,990 Coeterier May 2, 1944 2,431,077 Poch Nov. 18, 1947 2,460,609 Torsch Feb. 1, 1949 2,522,872 Heppner Sept. 19, 1950 2,555,850 Glyptis June 5, 1951 2,562,242 Pohle July 31, 1951 2,562,243 Pohle et al July 31, 1951 2,622,225 Linder Dec. 16, 1952 OTHER REFERENCES Article by Bowie, Proceedings of the Institute of Radio Engineers, vol. 36, N0. 12, December 1948, pages 1482- 1486. 

